Some Sustainability Aspects of Energy Conversion in Urban Electric Trains

Transcription

1 Sutainability 2010, 2, ; doi: /u OPEN ACCESS utainability ISSN Article Some Sutainability Apect of Energy Converion in Urban Electric Train Doru A. Nicola 1, Marc A. Roen 2, *, Cornelia A. Bulucea 1 and Contantin Brandua Faculty of Electromechanical and Environmental Engineering, Univerity of Craiova, Romania; dnicola@em.ucv.ro (D.A.N.); abulucea@gmail.com (C.A.B.) Faculty of Engineering and Applied Science, Univerity of Ontario Intitute of Technology, 2000 Simcoe Street North, Ohawa, Ontario, L1H 7K4, Canada Electrical Vehicle Department, ROMDATA AQ, Craiova, Romania; rintalctin@yahoo.com * Author to whom correpondence hould be addreed; marc.roen@uoit.ca; Tel.: ; Fax: Received: 1 April 2010; in revied form: 30 April 2010 / Accepted: 6 May 2010 / Publihed: 17 May 2010 Abtract: The paper illutrate ome apect of energy converion procee during underground electric train operation. Energy converion procee are explained uing exergy, in order to upport tranport ytem utainability. Lo of exergy reflect a lo of potential of energy to do work. Following the notion that life in Nature demontrate utainable energy converion, we approach the utainability of urban tranportation ytem according to the model of an ecoytem. The preentation tep baed on an indutrial ecoytem metabolim include decribing the urban electric railway ytem a an indutrial ecoytem with it limit and component, defining ytem operation regime, and aeing the equilibrium point of the ytem for two reference frame. For an electric train, exergy loe can be related to the energy flow during dynamic procee, and exergy converion in thee procee provide a meaningful meaure of the indutrial (i.e., tranportation) ecoytem efficiency. A a validation of the theoretical reult, a cae tudy i analyzed for three underground urban electric train type REU-U, REU-M, REU-G operating in the Bucharet Underground Railway Sytem (METROREX). The main experimental reult are preented and proceed, and relevant diagram are contructed. It i determined that there i great potential for improving the performance of rail ytem and increaing their utainability. For intance, power converter and efficient anti-kid ytem

2 Sutainability 2010, can enure optimum traction and minimum electricity ue, and the recovered energy in electric braking can be ued by other underground train, increaing exergy efficiency, although caution mut be exercied when doing o to avoid reducing the efficiency of the overall ytem. Keyword: urban electric tranportation; utainability; electric train; energy converion; exergy; induction motor; indutrial ecology Notation f (f N ) tator frequency (rated tator frequency) [Hz] f 2wR, f 2wS, f 2wT commutation function with two level i tranmiion ratio of gear reducer i d2 input current of VVVF inverter [A] i R, i S, i T output current of VVVF inverter [A] m railway vehicle ma [t] n rotor peed [rpm] p pole pair number of induction motor r pecific train reitance [dan/t] r p, i dc, r c pecific main reitance, pecific reitance caued by declivitie and pecific reitance caued by curve [dan/t] u d input voltage of VVVF inverter [V] u Nn zero-equence voltage [V] u, i, i r pace-phaor of tator voltage, tator current and rotor current (reduced to tator) u Rn, u Sn, u Tn output voltage of VVVF inverter [V] z traction motor number C2Q two quadrant converter D r wheel diameter [m] F total traction force [N] F 0 active motor force at rim [kn] F F braking force at rim [kn] G ad = m ad g adherent weight of railway vehicle [N] IT2N two level VVVF inverter L F C F input filter (network filter) L, L r, L u tator, rotor and ueful (magnetizing) cyclical inductance [H] M (M k, M N ) electromagnetic torque (maximum electromagnetic torque, rated electromagnetic torque) [Nm] M 2 ueful torque of traction induction motor [Nm] MAT traction aynchronou motor MW motor wagon

3 Sutainability 2010, P 0 R R R r U l, I l U (U N ) VVVF ξ = 1+g active power at rim [kw] total reitance to train running [N] phae tator winding reitance [Ω] rotor reitance reduced to tator [Ω] upply line voltage and RMS current of induction motor [V], [A] RMS phae tator voltage (RMS rated tator voltage) [V] Variable voltage variable frequency inertia contant of railway vehicle φ adherence coefficient η t tranmiion efficiency of gear reducer ω m = 2πpn/60 mechanical angular frequency [elect.rad/] ω r = ω ω m rotor angular frequency [elect.rad/] ω = 2π f (ω N ) tator angular frequency (rated tator angular frequency) [elect.rad/] σ leakage coefficient of induction machine (Blondel coefficient) Ω m Ψ (Ψ N ) Ψ, Ψ r mechanical angular peed of traction motor rotor [rad/] tator total flux (rated tator total flux) [Wb.turn] pace-phaor of tator fluxe and rotor fluxe (reduced to tator) 1. Introduction To addre meaningfully many of the problem facing humanity today, condition for the performance of utainable technical ytem mut be formulated. Real procee involving energy and material need to be linked to environmental impact a well a engineering deign and operation. Correpondingly, exergy concept can help undertand the efficiencie of electromechanically driven ytem and guide improvement effort [1,2]. Here, the applicability of exergy method to underground electrical railway tranport ytem i decribed. The merit of an urban electric tranportation ytem i baed not only on technical performance, afety, energy efficiency, economic and ocietal acceptance, but alo on environmental impact and exergy efficiency. Cot hould reflect value, which i aociated not with energy but rather with exergy and utainability. While energy i a meaure of quantity only, exergy i a meaure of quantity and quality or uefulne [3]. Thi tudy of underground electric train i baed on the concept that the efficiency reduction from large exergy detruction and the correponding long-term environmental degradation can be undertood and improved by viewing the electric train a an indutrial ecoytem. Thi reearch extend earlier work by the author [4,5]. Much other reearch ha been reported on the application of exergy in eeking utainable tranport. For intance, exergy ha recently been ued to ae the tranport ector in Greece [6] and the United Kingdom [7]. Some tudie have focued on applying exergy to rail tranport. For example, exergy ha been ued to invetigate the Italian railway tranport ytem [8] and electric tranport device uing fuel like liquefied natural ga [9]. Increaing effort are being expended to enhance the utainability of tranportation technologie and ytem. For intance, approache to and iue in utainable tranport ytem have been

4 Sutainability 2010, explored [10,11], a have policy requirement to achieve utainable urban tranport [12]. Reearch to improve rail tranport utainability ha alo received increaed focu, with initiative reported on innovative lightweight tranit technologie [13] and combining electric vehicle with intermodal tranport [14]. Some of thee effort have focued on reducing the environmental impact of rail tranport by reducing emiion of greenhoue gae and other pollutant [15]. Following the above context, thi paper decribe the electric deign of urban rail tranportation ytem a well a their characteritic and performance, conidering their drive ytem with induction motor upplied at variable frequency with controlled tator flux. After modeling the main traction equipment, i.e., induction motor and variable voltage variable frequency (VVVF) inverter, the deign of a tructural diagram of the main electric circuit on urban electric train i preented, together with the tructural model of ueful movement. Finally, on the bai of a particular cae, the energy conumption i validated in thi paper correponding to a complete cycle (i.e., acceleration, contant running peed and deceleration) for the analyzed underground train. 2. Energy Converion in Urban Electric Train At the tart, we adopt a dualit view [16], in which the tranportation ytem i taken to be urrounded by two environment: technical and ecological. The electric tranportation ytem i cloed within the technical urrounding, but open within the ecological environment. Modern urban electric railway train are upplied by a d.c. contact line and are equipped with three-phae induction motor and variable voltage variable frequency (VVVF) inverter [17]. Since electric drive ytem are ued with tatic converter and traction induction motor, with appropriate control thee machine can realize both traction and electric braking regime of electric traction vehicle. Utilization in electric traction of an induction motor with a rotor quirrel cage i poible only in the condition of a three-phae upply ytem with controlled variable frequency and r.m.. voltage [18]. Thi upply type i achieved with a machine-ide converter (CM), uually a VVVF inverter (IT) with two or three level (Figure 1). The working mechanim (ML) repreent the mechanical part of the motor electric vehicle that it i located between a movement tranformer (reduction gear) and a movement converter (wheel + rail) which tranform rotation into tranlational movement. But, to commence, we need to know in what way electric traction drive ytem baed on tatic converter lead to an improvement in electric train performance, regarding both optimum traction characteritic (in term of energy ue) and high exergy efficiency in train operation, particularly for electric braking with energy recovery Operation at Variable Frequency of Induction Machine An indutrial ecoytem doe not have a ingle equilibrium point [19]. Rather, the ytem move among multiple table tate [20]. The dynamic regime in electric train operation, e.g., tarting or braking procee, can be viewed a repreenting the indutrial ecoytem movement among point of equilibrium.

5 Sutainability 2010, Figure 1. Principal cheme of main electric circuit of urban electric train fed by a d.c. contact line. Vehicle regulation peed i determined examining the tatic converter (VVVF) and electric machine a an aembly [21]. The traction motor peed regulation i baed on tator voltage and frequency variation, o that to achieve high exergy efficiency, the firt requirement of the train control ytem concern paing of the motor operation equilibrium point from one mechanical characteritic to another. In the range of frequencie lower than the rated frequency, f < f N, in order to enure a contant level of inductor machine tator flux Ψ = Ψ N, we have to imultaneouly modify the frequency f and the upply voltage magnitude U o the upply r.m.. voltage U varie with the frequency f according to U (f ) = R I + j2πf Ψ Sn, where R i the tator reitance and Ψ the tator flux. In the cae conidered ubequently of an induction machine upplied by a variable frequency voltage ource, when the rated peed i reached (i.e., the induction motor i upplied at U N and f N ) a further increae in peed i poible only by increaing the tator frequency magnitude over the rated frequency f > f N. Note that becaue of the voltage retriction on both converter and induction machine winding inulation conideration, the tator voltage i limited and maintained at a contant magnitude U = U N for the entire high frequency domain, and the induction machine operate in weakened flux condition [17]. From an exergetic viewpoint it i noted that, becaue the tator flux and pulation exhibit an invere proportionality relation Ψ = U N /ω, the machine torque capability i trongly affected, and the exergy detruction can not be avoided. We conclude that the induction machine upplied from a variable frequency and voltage ource operate with full field ψ = ψ Sn = cont. in the low frequency range f f N and with a weakened field (ψ < ψ Sn ) in the increaed frequency domain f > f N (when the upply r.m.. voltage remain contant (U = U N = cont.) Electric Train with Induction Motor Operating at Variable Frequency and Controlled Flux The operation at variable frequency with controlled flux i preceded for induction motor in drive ytem with vectorial control [22]. The vectorial regulation and control method i baed on pace phaor

6 Sutainability 2010, theory, taking into conideration the control of both the flux and the induction machine electromagnetic torque M. In principle, the tator current pace phaor i decompoed into two perpendicular component (a flux component and a torque component) which are eparately controlled. In thi paper we analyze the teady-tate inuoidal regime of variable frequency operation with controlled tator flux. Note that the theoretical development preume an induction machine with contant parameter, without iron loe and without aturation of the magnetic core. In the teady-tate the following relation can be derived for the tator current component [23]: ψ 1- σ ψ r r x = + ω σl L σl R r r Lr R r + ω σ ωrσ r r I I y 1- σ = σ L R L R ψ ω σl R L r r r + ωrσ r r The abolute value of the tator current can be determined with the formula I = (I x 2 + I y 2 ) 1/2. Within the ecological framework, the electromagnetic torque M i again related to the ytem output exergy. We can expre M in complex coordinate axe ytem (oriented on ψ ) a (1) M = 3p Im{ I ψ } = = 3p Im{( Ix + ji ) ψ } = y = 3p ψ I y * (2) Subtituting I y from (1), the torque relation become 1- σ M = 3p σ R ω σl 2 ψ ω σl R L r r r + r r r (3) If the tator flux ψ i contant, the electromagnetic torque magnitude depend on the rotor current pulation ω r but not the tator upply frequency f. The torque curve M = f(ω r ) at ψ = cont. i not linearly dependent on ω r, having two ymmetrical extreme: M R r = 0 ; ωrkψ = ± ; ωr σ Lr 3p 1- σ M = M( ) = ± ψ 2 L 2 kψ ω rk ψ σ The dependence of M = f(ω r ) at ψ = cont. i hown in Figure 2. In a teady-tate regime, a ytem table operation (with M/ ω r > 0) i performed only on the acendant zone of the characteritic M = f(ω r ) in Figure 2 and correpond at mall rotor pulation to the condition ω r ω rkψ. The mechanical characteritic family M = f(n) of the induction motor operating at ψ = cont., for different tator frequencie f are hown in Figure 3. (4)

7 Sutainability 2010, Figure 2. Torque characteritic M = f (ω r ) at controlled tator flux. Figure 3. Mechanical characteritic M = f(n) at ψ = cont. for different frequency value f f N. The contant tator flux magnitude ψ for any tator frequency f and torque M (repectively, any rotor pulation ω r ) impoe an exact control of either the upply voltage U or the upply current I. We ee again an analogy between thi electrical ytem and an ecoytem. An appropriate technical ytem control mut be achieved for reducing exergy detruction when the equilibrium point pae from one table tate (repreented by the operation point on a certain mechanical characteritic) to another table tate (on another mechanical characteritic). Thi obervation implie the ytem control need to be aeed next Modeling of Electric Train Obervation of nature ugget we need to identify modalitie for optimum train control to minimize energy conumption and maximize exergy efficiency. We know the hape of the ideal traction characteritic [17]. The real traction characteritic identifie if the train control i appropriate. To achieve vehicle control require an analyi baed on tructural diagram.

8 Sutainability 2010, Firt, we model the traction induction motor, where electromechanical energy converion occur. A a complex electromechanical ytem, the induction motor can be conceptually decompoed into electromagnetic and mechanical ubytem. Between thee functional part, the electromagnetic torque M and the rotor mechanical peed Ω m interact a internal variable. The induction motor electromagnetic part can be decribed by the following equation [17]: d ψ = u - R i dt dψ' r = j p Ωm ψ' - R' r r i' r dt - Lu ' ' - Lu ψ ψ ψ ψ r r i = L' r ; i' = L r σl σlr 3 * M = p Im{i ψ } 2 Here, u denote the tator voltage vector; i the tator current vector; i r the rotor current vector; Ψ the tator flux vector; Ψ r the rotor flux vector; L u the magnetizing inductance; L the tator inductance; L r the rotor inductance; p the number of pole pair; R the tator reitance; R r the rotor reitance and 2 σ = 1- L the motor leakage coefficient. u L Lr Uing Equation (5), the tructural diagram and the mak block of the induction motor electromagnetic part can be repreented (Figure 4). Figure 4. Structural diagram and mak block for induction motor electromagnetic part. (5)

9 Sutainability 2010, The tructural diagram of the electromagnetic ubytem can be coupled both with the tructural diagram of the VVVF inverter through the input variable u and output variable i and with the tructural diagram of the mechanical ubytem via the input quantity Ω m and the output quantity M. To model the machine-ide converter, uually a voltage-ource inverter, commutation function are ued in two or three level, repectively [17,18]. A tatic converter can be viewed for modeling a a black box with input/output characteritic through the commutation function. Analying the topology of a three-phae voltage-ource inverter with two level and uing the commutation function f 2w (in two level), be written a: = f 2w 1 0, for each arm of converter, equation for the inverter model (without loe) can u u Rn RN = ud f 2wR ; usn = ud f 2wS ; utn = ud f 2wT; id2 = ir f 2wR + is f 2wS + it f 2wT; = urn - unn ; usn = usn - unn ; utn = utn - unn ; urn + usn + utn unn = ; 3 With thi ytem of equation, the tructural diagram and mak block of the three-phae voltage-ource inverter with two level are obtained (Figure 5). Figure 5. Structural diagram and mak block for three-phae voltage ource inverter with two level. (6) To highlight the exergetic dynamic approach of the ytem, a vehicle ueful movement model i alo neceary [17]. Here, we conider a motor electric vehicle with ma m[t] and inertia contant ξ having a pecific train reitance r[dan/t]. If movement occur due to the action of ueful torque M 2 (identical), developed by z vehicle traction motor of the motor electric, then the ueful movement can be decribed a:

10 Sutainability 2010, i v = (F - R)dt ; Ωm = v; x = v dt m ξ Dr 2 F = z i ηt M2 Dr R = (rp (v) ± i (x) + rc (x)) m 10 de (7) which allow the contruction of the tructural diagram of ueful movement (Figure 6). For the mak block, we conidered a an input quantity the torque M and a an output quantity the peed Ω m, i.e., the time-varying quantitie during ueful movement. Figure 6. Structural diagram and mak block of ueful movement. UM Subequently, diagram of train running peed v(t) and ditance x(t) can be developed. Modification to vehicle ma and dependence i de (x) or r c (x), pecific to a vehicle or route, can be incorporated, yielding an exact model for all operating condition. Figure 7. Structural diagram of main electric circuit.

11 Sutainability 2010, We began our tudy conidering the main circuit of electric train fed from a d.c. contact line (Figure 1) and have obtained it tructural diagram (Figure 7). Thi information upport the aim of optimizing vehicle control to obtain a high exergy efficiency. 3. Cae Study In thi cae tudy we conider an urban electric train with two motor wagon (MW) which are elatic coupled and which are referred to a MW + MW [23]. The vehicle electric cheme (Figure 8) i in variant V2, which i defined by the coefficient K = 2/2, indicating two tatic converter, each upplying two traction bi-motor. On the bai of the previou tructural diagram, the tructural diagram for the urban electric vehicle viewed a a ytem i aeed in Figure 9. The electric traction cheme with a power upply from a d.c. network ha the following element [17,23]: a current connector to a third rail; a loading contactor and a loading reitor; a rapid automatic circuit breaker; an input circuit (known a a LC filter); a voltage and frequency converter; electric traction motor; a braking chopper and a braking reitor and a hunt; and electric coupling for the wagon. From the technical point of view, the electric traction cheme preented here meet the criteria of vehicle operating afety and traction cheme reliability. An undertanding of the utainability of the electric tranportation ytem can be obtained with an analyi of the exergy converion chain within the electric tranportation ytem operation regime. Figure 8. Electric traction diagram in variant K = 2/2.

12 Sutainability 2010, Figure 9. Structural diagram of urban electric vehicle MW + MW, with K = 2/2. Here, the train operation i briefly decribed. After the connector coupling, the circuit breaker and contactor witching, the control of the unit inverter i determined. The traction induction bi-motor group are upplied from the three-phae VVVF inverter. Conequently, the urban electric train i able to operate. At the minimum adjuted frequency f min = f n. the traction motor M 1 and M 2 on the firt unit MW A are immobile. Similarly M 1 and M 2 are placed on the econd unit MW B. When the frequency exceed that value, the motor tart to move, with the operation following the frequency mechanical characteritic correponding to the minimum upply frequency. The electric train accelerate at a contant traction torque, with operation on the mechanical characteritic at up to f n, when U 1 = U 1n and then, over f n, at contant power. The three-phae traction induction motor revere their rotation via a imple upply commutation, by tator phae ucceion witching. The motor have identical characteritic for both rotation ene. It i emphaized that exergy iue prefer the vehicle electric motor operate at the deigned rating power for a running peed range a long a poible [17]. Hence, the active power at rim P 0 i contant and the active force F depend on the peed according to P0[kW] F[kN] = 3.6 (8) v[km/h] On bai of equation (8), a hyperbole form of the active force F reult, a hown in Figure10. Figure 10. Ideal mechanical characteritic.

13 Sutainability 2010, From the viewpoint of exergy efficiency and environment iue, a pecial apect in cae of the nonautonomou vehicle with electric traction i repreented by the braking regime, particularly electric braking. Since electric driving ytem with tatic converter and traction induction motor are ued, electric braking with the ame motor can be realized with appropriate control for electric traction vehicle. For a pecified running direction, paing from the traction regime to the electric brake regime correpond to a ign change of the active force F. Clearly in the traction regime the vehicle peed v and the active force F have the ame ign, while in the braking regime v and F have oppoite ign. To perform electric train braking, the traction induction motor pa into the electric generator regime, by decreaing control of the upply voltage frequency. The electric traction machine operate in the generator regime on the mechanical characteritic M = f(n) in quadrant II and IV (Figure 2 and 3), repectively. In that ituation, which i complex from the viewpoint of power circulation, the inverter provide the reactive energy for the traction machine in the generator regime uing the capacitor battery from the LC circuit and through the recovery diode group, and the electrical machine upplie power like an induction generator into the d.c. network. Thi recovered energy i ued by other operating train. Thu the exergy efficiency i high. The d.c. network capacity to receive thi electric energy i continuouly checked by the vehicle control ytem. If the input circuit voltage exceed 1.2U d (i.e., over = 900 V), braking chopper are automatically activated, which realize an electric rheotatic brake regime. The braking reitance allow energy diipation (by the Joule effect) of the uncirculated electric energy. Then, a high energy efficiency of the brake reitance i obtained, but the ytem exergy efficiency i ignificantly decreaed. The environmental impact from the electric braking regime, particularly with recovered energy, by hifting the traction machine to an electric generator regime, are coniderably reduced. 4. Experimental Reult The Bucharet Underground Railway Sytem (METROREX) ha train equipped with traction induction motor MAB T 1, MAB T 2 and MAB T 3 produced by the Electroputere Factory in Craiova. The following railway vehicle type are conidered: urban electric train heavy implementation REU-G with a weight of 36 t/wagon, urban electric train medium implementation REU-M, with a weight of 25 t/wagon, and urban electric train light implementation REU-U, with a weight of 15 t/wagon. For REU-G, experimental traction characteritic are plotted in Figure 11, howing the variation of characteritic are imilar, uggeting the goal of minimum energy conumption in the traction regime i accomplihed. Data are provided in Table 1.

14 Sutainability 2010, Figure 11. Variation of Rp, Po, Fo and v for REU-G. REU-G 10*Rp[kN] (1/10)*Po[kW] Fo[kN] v[km/h] Table 1. Value of Rp, Po, Fo and v for REU-G. Rp [N] Po [kw] Fo [kn] v [km/h] Readin g number 1, , , , , , , , , , , , , , , , , Experimental electric braking characteritic for three train type REU-U, REU-M, REU-G ued by the Bucharet Underground Railway Sytem are preented in Figure 12 and Table 2.

15 Sutainability 2010, Figure 12. Variation of force F F and peed v in electric braking regime for vehicle REU- U, REU-M, REU-G. REU-U REU-M REU-G v[km/h] Table 2. Experimental electric braking characteritic for three train type. REU-U F F [kn] REU-M F F [kn] REU-G F F [kn] v [km/h] Readin g number The Urban Underground Railway Tranportation Sytem in Bucharet i a network of four line (M1, M2, M3 and M4) with 49 train tation and a total length of 67 km. Train travel between 5 a.m. and 11:30 p.m., with an average frequency of 10 minute.

16 Sutainability 2010, In our calculation we aume the following: a 1,560 m length between conecutive tation, a running time between conecutive tation of 96, with 20 tranient tarting (10 at contant force and 10 at contant power), 60 of contant peed running and 16 electric braking, and electric active energie a in Table 3, following the electric active power P value in Figure 13. During deceleration (interval ), the electric active power (3,000 kw) correpond to recovered energy or heat, depending on electric brake type. An analyi of the recovered energy indicate that on a railway egment at leat three train mut be in operation (two at peed higher than 10 m/ and one in a uniform electric brake regime). So, for high exergy efficiency, the number of train operating on a rail egment hould be n = [3,000/2,400] Dicuion The train cae tudy conidered yield particularly ueful reult in the traction regime. Uing tructural diagram and high-performance converter, appropriate vehicle control can be achieved o the train experimental dynamic characteritic follow the theoretical mechanical characteritic. Then, the energy and exergy efficiencie are equal. Power converter and efficient anti-kid ytem enure optimum traction and minimum energy ue. Table 3. Data for railway ytem. Parameter Value Railway length 1,560 m Cycle time 96 Train number 1 Electric frame number 3 Number of wagon 3 2 = 6 Number of motor 6 4 = 24 Intalled power, P 2,400 kw Running peed, v o 20 m/ Acceleration time, t ac 20 Uniform running time, t ur 60 Deceleration time, t dc 16 Electric energy in acceleration regime, E ac 36,000 kw (10 kwh) Electric energy in uniform running regime, E ur 144,000 kw (40 kwh) Electric energy in deceleration regime, E dc 48,000 kw (13 kwh)

17 Sutainability 2010, Figure 13. Variation with time of peed and upplied electric active power for a complete cycle: acceleration, contant peed running and deceleration. v[m/] 20 m/ P[kW] 2400kW 3000kW t[] Concerning train electric braking, further reearch i needed on the tranportation ytem internal exergy conumption. Actual technique allow driving ytem to be implemented on the bai of variable voltage and frequency tatic converter and induction motor, which facilitate improved electric braking, even with energy recovery. In that operating regime, the vehicle provide energy to the d.c. network through an inverter. The recovered energy i ued by other underground train, increaing exergy efficiency. Another reult related to exergy efficiency improvement relate to the railway tranportation ytem traffic intenity and may eem paradoxical. At preent, the energy recovered from electric braking can be provided only to running train in the tranportation ytem. A the number of running train in the ytem increae, the exergy efficiency i high if the recovered electric energy i properly ued. But with few running train, if the third rail voltage exceed 900 V, the rheotatic brake regime i automatically controlled and the recovered electric energy i tranformed by the Joule Effect to heat. Thi i an unfavorable ituation, with adequate energy efficiency, but a low exergy efficiency. Electric braking with energy recovery hould be compulory in the long run in electric tranportation ytem to improve their utainability, and the elaticity of reverible traction ub-tation equipment hould be taken into account. The reult are expected to help make electrical tranportation ytem more utainable. 6. Concluion Energy ytem involving converion chain procee often are highly irreverible and have low exergy efficiencie. Within an Indutrial Ecology framework there i great potential for improving the performance of uch technical ytem and increaing their utainability. For intance, power converter and efficient anti-kid ytem can enure optimum traction with minimum electricity ue, while energy can be recovered effectively via electric braking for ue in other underground train, increaing exergy efficiency. Further reearch i needed on the tranportation ytem internal exergy efficiencie and conumption related to train electric braking. Care mut be exercied when implementing exergy efficiency improvement due to potential negative effect related to railway tranportation ytem traffic intenity, which can lead to an adequate energy efficiency but a low

18 Sutainability 2010, exergy efficiency. Electric braking with energy recovery hould be compulory in the long run in electric tranportation ytem to improve their utainability, and the elaticity of reverible traction ub-tation equipment hould be taken into account. If applied, the reult of thi invetigation can help make electrical tranportation ytem more utainable. Acknowledgement Financial upport wa provided by the Natural Science and Engineering Reearch Council of Canada. Reference and Note 1. Roen, M.A. A Concie Review of Energy-Baed Economic Method. In Proceeding of the 3rd IASME/WSEAS International Conference on Energy & Environment, Cambridge, UK, February 2008; pp Allenby, B.R. Indutrial Ecology: Policy Framework and Implementation; Prentice-Hall: Old Tappan, NJ, USA, Dincer, I.; Roen, M.A. Exergy: Energy, Environment and Sutainable Development; Elevier: Oxford, UK, Roen, M.A.; Bulucea, C.A. Aeing Electrical Sytem via Exergy: A Dualit View Incorporating Technical and Environmental Dimenion. In Proceeding of the 6th WSEAS International Conference on Engineering Education (EE 09), Rhode, Greece, July 2009; pp Nicola, D.A.; Roen, M.A.; Bulucea, C.A.; Brandua, C. Sutainable Energy Converion in Electrically Driven Tranportation Sytem. In Proceeding of the 6th WSEAS International Conference on Engineering Education (EE 09), Rhode, Greece, July 2009; pp Koroneo, C.J.; Nanaki, E.A. Energy and Exergy Utilization Aement of the Greek Tranport Sector. Reour. Coner. Recyc. 2008, 52, Gaparato, A.; El-Haram, M.; Horner, M. A Longitudinal Analyi of the UK Tranport Sector, Energ. Policy 2009, 37, Federici, M.; Ulgiati, S.; Baoi, R. A Thermodynamic, Environmental and Material Flow Analyi of the Italian Highway and Railway Tranport Sytem. Energy 2008, 33, Dipenza, C.; Dipenza, G.; La Rocca, V.; Panno, G. Exergy Recovery during LNG Regaification: Electric Energy Production Part One. Appl. Therm. Eng. 2009, 29, Potter, S. Exploring Approache toward a Sutainable Tranport Sytem. Int. J. Sutain. Tranp. 2007, 1, Litman, T.; Burwell, D. Iue in Sutainable Tranportation. Int. J. Glob. Environ. Iue 2006, 6, Hull, A. Policy Integration: What Will It Take to Achieve More Sutainable Tranport Solution in Citie. Tranport Policy 2008, 15, Siu, L.K. Innovative Lightweight Tranit Technologie for Sutainable Tranportation. J. Tranp. Sytem Eng. Inform. Technol. 2007, 7,

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